The present invention relates to blades for a steam turbine rotor. More specifically, the present invention relates to a light weight blade for use in the transition zone of a low pressure steam turbine that substantially reduces the centrifugal stress on the blade root and the rotor groove in which it is disposed.
The steam flow path of a steam turbine is formed by a stationary cylinder and a rotor. A large number of stationary vanes are attached to the cylinder in a circumferential array and extend inward into the steam flow path. Similarly, a large number of rotating blades are attached to the rotor in a circumferential array and extend outward into the steam flow path. The stationary vanes and rotating blades are arranged in alternating rows so that a row of vanes and the immediately downstream row of blades forms a stage. The vanes serve to direct the flow of steam so that it enters the downstream row of blades at the correct angle. The blade airfoils extract energy from the steam, thereby developing the power necessary to drive the rotor and the load attached to it.
The amount of energy extracted by each row of rotating blades depends on the size and shape of the blade airfoils, as well as the quantity of blades in the row. Thus, the shapes of the blade airfoils are an extremely important factor in the thermodynamic performance of the turbine and determining the geometry of the blade airfoils is a vital portion of the turbine design.
As the steam flows through the turbine its pressure drops through each succeeding stage until the desired discharge pressure is achieved. Thus, the steam properties--that is, temperature, pressure, velocity and moisture content--vary from row to row as the steam expands through the flow path. Consequently, each blade row employs blades having an airfoil shape that is optimized for the steam conditions associated with that row. However, within a given row the blade airfoil shapes are identical, except in certain turbines in which the airfoil shapes are varied among the blades within the row in order to vary the resonant frequencies.
Designing a steam turbine blade is made difficult by the fact that the airfoil shape determines, in large part, the mechanical strength of the blade and its resonant frequencies, as well as the thermodynamic performance of the blade. These considerations impose constraints on the choice of blade airfoil shape so that, of necessity, the optimum blade airfoil shape for a given row is a matter of compromise between its mechanical and aerodynamic properties.
This situation is exacerbated in the transition zone of a low pressure steam turbine, such as that used in a boiling water reactor nuclear power plant, due to the high moisture content of the steam flow in that zone. Such high moisture content results in erosion and corrosion of the components exposed to the steam flow. Most importantly, for present purposes, this erosion and corrosion can weaken the blade, especially in the root area, as well as the rotor groove to which the blade is secured. Since the blade root and rotor groove are subjected to high stresses due to centrifugal force, the erosion and corrosion can result in cracking.
Consequently, it is important that the weight of the blade be minimized, thereby minimizing the centrifugal force to which the blade root and rotor groove are subjected during operation.
It is therefore desirable to provide a row of steam turbine blades that achieve adequate performance using very light weight airfoils.